1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus and an image data generating method, and more particularly, to an ultrasonic diagnostic apparatus and an image data generating method which obtain ultrasonic image data with high resolution by detecting harmonic component of an ultrasonic reflected wave from a subject.
2. Description of the Related Art
An ultrasonic diagnostic apparatus serves so as to emit an ultrasonic wave generated by an ultrasonic transducer built in an ultrasonic probe into a subject, receive a reflective wave generated due to a difference in sound impedances of subject tissues by the ultrasonic transducer and display it on a monitor.
An ultrasonic diagnosis is widely used for a function test of a heart or the like, or a morphological diagnosis of a variety of organs since a two-dimensional image can be easily observed in real time by simply contacting an ultrasonic probe to the body surface. Also, since the ultrasonic diagnosis is free from radiation exposure occurring in a diagnosis with the aid of an X-ray diagnostic apparatus or an X-ray CT scanner, it has a large number of advantages, for example, not only diagnosis of a heart, an abdomen, a mammary gland, and a urinary organ but also repetitive diagnosis of a fetus in the obstetrical field, in addition to usability at bed side thanks to its small size.
In the ultrasonic diagnosis, an image data is generated by emitting an ultrasonic pulse, having its center frequency at a predetermined frequency selected depending on a diagnostic portion, into a subject and receiving an ultrasonic reflected wave having substantially the same frequency as the ultrasonic pulse.
In contrast to this, in recent years, a new imaging technology called a Tissue-Harmonic-Imaging method (hereinafter, referred to as a THI) has been developed and begins to come into wide spread use in a clinical field. With this imaging method, an ultrasonic non-linear phenomenon generated in tissue of a subject is effectively used. For example, when an ultrasonic pulse having its center frequency f0 is emitted into the subject, a second harmonic component 2f0 newly generated due to the non-linear phenomenon of the tissues of the subject is selectively received and converted into an image.
The harmonic component is newly generated with respect to the ultrasonic pulse having a fundamental frequency (hereinafter, referred to as the fundamental component) emitted into the subject and its generation depends on the property of the subject tissue, a transmission distance to a reflective portion, and the ultrasonic intensity at the reflective portion. Hence, receiving sensitivities of a multiple reflection wave and a side lobe generated between the ultrasonic probe and an organ border and serving as the major factor of artifact in a conventional ultrasonic image can be reduced relative to that of the fundamental component. Accordingly, the THI using the harmonic component allows clear image data including little artifact to be obtained (see, for example, Japanese Patent Application (Laid-Open) No. 10-179589).
In the THI, a harmonic component is generally extracted from an ultrasonic reflected wave serving as a mixture of the fundamental and harmonic components with the filtering method. However, in the case where the fundamental and harmonic components have broadband spectra, since respective parts of the components overlap with each other, accurately extracting only the harmonic component with the filtering method is difficult.
As a method for extracting the harmonic component from such a broadband ultrasonic reflected wave, a pulse inversion method has been developed. With this method, in the case where an ultrasonic wave is transmitted/received in a predetermined direction, only the harmonic component is extracted by alternately transmitting two kinds of ultrasonic pulses having mutually different polarities so as to cancel their fundamental components by adding receiving signals obtained on this occasion to the ultrasonic pulses. The pulse inversion method is established by focusing attention on the fact that the waveform of the harmonic component is formed in proportion to the square of the amplitude of the waveform of the fundamental component and is based on the property of the ultrasonic wave that inverse of the polarity of the ultrasonic pulse causes its fundamental component to be likewise inversed but its harmonic component not to be inversed (see, for example, Japanese Patent Application (Laid-Open) No. 9-164138).
In the meantime, the harmonic component generated upon reflection of a transmission ultrasonic wave having a predetermined frequency on the tissue of the subject is significantly smaller than the fundamental component. For example, it is confirmed that the sensitivity of the second harmonic component is generally lower than that of the fundamental component by at least 20 dB although depending on the intensity of the transmission ultrasonic wave.
In addition, in the course of receiving the harmonic component reflected at the tissue of the subject by the ultrasonic probe, an ultrasonic attenuation due to absorption in the tissue depends on an ultrasonic wave frequency. For example, it is known that the ultrasonic attenuation of the foregoing second harmonic component is about two-fold in decibel unit when compared to that of the fundamental component.
With such a reason, THI image data generated on the basis of the harmonic component has a poorer S/N ratio than that of an image data generated mainly from the conventional fundamental component (i.e., an image data generated from the fundamental and harmonic components), and in particular, it is difficult to obtain a high-resolution image data of a deeply existing organ remote from the ultrasonic probe.
While two methods for improving the S/N ratio of the receiving harmonic component are provided: one for reducing noises (N) in a receiving circuit and the other for improving the receiving intensity of a signal component (S), since the former one has already reached the limit for the duty, the S/N ratio cannot be improved without the help of the latter one.
While the receiving sensitivity of the harmonic component could be improved by increasing a transmission sound output of the apparatus, when the transmission sound output is increased with a method similar to the conventional one, it is difficult to comply with the heat generation and sound output regulations of the ultrasonic probe set while taking into account the safety of the subject.